The production of hollow articles by a process known as "rotational molding" is well known in the art. See, for example, U.S. Pat. No. 2,881,476 to Page, U.S. Pat. No. 3,134,140 to Knowles, U.S. Pat. No. 3,703,348 to Pivar and U.S. Pat. No. 3,822,980 to Graeper. In rotational molding a charge of resin such as a powder of thermoplastic material is placed in the cavity of a mold. The mold is heated and rotated, either partially or fully, to tumble the charge of resin, which fuses to the mold wall with gradually increasing thickness to form the article. In the molding apparatuses of Page, Knowles and Pivar, the mold is directly heated by flames of gas, whereas the mold in the apparatus described in the Graeper patent is heated and rotated in an oven.
The rotational molding of large tanks can be achieved. Such large tanks may be used for many different purposes, but a particular advantage of a plastic tank is its utility in the food processing industry. Large plastic tanks, however, are by themselves difficult to move about when containing a liquid. Fork lift equipment may be used to pick up a tank placed on a skid into which the pick-up fork of the lift can be inserted. Such skid, however, when formed of wood may not be acceptable in a food processing plant and accordingly, plastic tanks are often provided with suitable legs to raise the tank bottom to a level where a fork lift can be used to move the tank.
The use of tank legs, for example, at four bottom corners of a rectangular tank, has the advantage that a fork lift can be inserted below the tank bottom between any two legs and thus form any side. The tank legs being made simultaneously with the rotational molding of the tank, are as a result hollow, thus forming corner located recesses inside the tank.
These recesses may at times present an irregular internal surface with ridges as a result of an irregular build-up of the tank wall at the legs. Such irregularity tends to arise particularly when, during the rotational molding, preferential orientations are employed to increase the thickness of the tank wall at the legs for improved strength. These irregularities on the surface of the wall of the recesses tend to collect and retain food materials, thus leading to bacteria formation unless great care is exercised to clean the recesses. Techniques are available to smooth the wall surface, but aside from the burden such wall smoothing operation would add to the rotational molding of a tank with legs, one still is left with a recess, which if not properly cleaned, presents a potential source of poor hygiene.
Rotationally molded plastic tanks for use in food processing plants, therefore, are commonly required to have a smooth walled interior without small recesses. When hollow tank legs are added, techniques have been employed to close the recesses in the legs. One approach involves forming the legs of solid plastic during the molding process. This, however, requires an undesirably large amount of plastic. Such heavy wall thickness for the tank legs is also difficult to obtain since, as the wall thickness increases, the wall itself becomes an insulator and, at least in the technique of heating the mold from an external heat source, eventually inhibits sufficient fusing of thermoplastic material to form a solid leg.
Another technique involves welding legs onto the tank after the tank with a smooth bottom has been molded. A plastic weld zone, however, often does not have the desired strength and welded-on legs tend to break off during tank manipulation. One may, for example, imbed a metal fitting in the tank bottom to add strength in the subsequent attachment of a leg. Such fitting can be added by utilizing a flange which is spaced above an opening in the mold by a suitable spacer. The spacer fits over and closes the mold opening and a bolt, connected to the spacer, protrudes externally through the mold opening. During rotational molding, plastic fuses into the space between the flange and the mold wall as well as over the flange to thus imbed the latter in the tank bottom. This technique can be used to improve attachment of tank legs, but the strength of the attachment depends on the plastic portion which imbeds the flange. This technique usually results in inadequate strength of attachment of the tank legs and since it involves special high strength fittings, tends to be uneconomical.